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How Can Plant Growth-Promoting
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By R. Murphey Coy and David W. Held, Ph.D., Department of Entomology and Plant Pathology, Auburn University, Auburn AL (or plant-growthpromoting rhizobacteria) are not new or even novel, yet their interactions with plants may lay the groundwork for novel solutions to several different issues in turfgrass culture. Plant-growth-promoting rhizobacteria (PGPR) are non-pathogenic, beneficial, free-living soil- and root-inhabiting bacteria that are able to colonize seeds and roots (rhizosphere). Bacterial inoculants have been extensively used in annual field crops (usually as a seed coating or treatment), yet little attention has been given to use in perennial crops like turfgrass.
PGPR
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What Are Rhizobacteria? • These bacteria colonize plant roots. • There are multiple species or strains (like bacterial cultivars). • They can be good or bad for plants. • “Good” rhizobacteria can induce benefits in plants. • Some are not formulated for application to turfgrass. 22
Products marketed with PGPR are formulated as single species of bacteria or as blends of multiple species or strains. PGPR products may be available as a liquid or granule, and they are compatible with most chemicals already used. For example, Bayer Environmental Sciences introduced Nortica®, Bacillus firmis, a single-species PGPR granular product for enhanced growth and nematode control in turfgrass. Other PGPR products include Growth Products Companion®, Brandt Noculate®, Nocu-tain®, Nocu-Gro®, Nocu-Start®, Roots Ecoguard®, Flexx® and Healthy Turf®. Often, the bacteria are formulated with a complement of plant nutrients. What happens when PGPR are applied? The rhizosphere is the layer of soil that is influenced by the plant root, and it has a greater density of organic carbon and bacteria than the rest of the bulk soil. This allows for the plant’s roots to secrete root exudates and metabolites that can be used as plant nutrients. The bacterial community is richer around the rhizosphere, so competition exists between microbes for limited soil nutrients and space that can be colonized on the root. Therefore, to have a positive impact on the plant, bacterial inoculants must survive inoculation onto the roots, multiply and colonize the developing root system.
Photo 1. Average root system after five weeks of Control and PGPR-treated Tifway.
In many cases, PGPR are both endophytic (capable of living within the plant tissue) and epiphytic characteristic (capable of living on plant surfaces). There is also some evidence that solutions containing PGPR may also have direct negative impacts on pests such as plant parasitic nematodes in the soil water. The mechanisms of PGPR on plants are not well understood, but studies have shown that PGPR somehow mediate the plant’s ability to tolerate abiotic and biotic stress. For example, our recent work with AU blends of PGPR and